Coherent optical communications enable data to be transmitted over long haul optical transmission networks, typically greater than 2,000 km, at high data rates. A coherent detector requires that a received phase-modulated optical signal is digitized. Digitizing the received signal requires a high-speed analog-to-digital converter (ADC). However, an increase in serial ADC sampling rates is difficult to implement for single-wavelength coherent detection at data rates greater than 100 Gbit/s.
A super-channel transmission technique is an evolution in dense wavelength division multiplexing (DWDM), in which multiple optical carriers are combined on a single super-channel to realize a higher data rate in total, and is brought into service in a single operational cycle. Rather than a single wavelength channel, e.g., of 100 Gbit/s, a super-channel uses a multi-wavelength signal, in which each wavelength operates as a sub-channel at the maximum data rate permitted by ADC components.
One notable difference between the super-channel and conventional wavelength division multiplexing (WDM) is a size of a gap between frequencies of different sub-channels. The super-channel can decrease the size of the gap between wavelengths of the sub-channels so that the super-channel effectively operates as a single wideband channel, in terms of adding, dropping, and routing data. Various techniques can be used to reduce the sub-channels frequency gap to the MHz range. These techniques include orthogonal-band-multiplexing (OBM), orthogonal frequency division multiplexing (OFDM), no guard interval (NGI)-OFDM, Nyquist WDM, and multi-channel equalization (MCE)-WDM.
However, the small size of the gap between frequencies of different sub-channels can cause inter-channel interference (ICI) in the signals transmitted on the sub-channels. The ICI is considered as additional noise in the received signals and the received data has to be recovered subject to sometimes prohibitively large total noise. To avoid the ICI, conventional systems filter the transmitted signal, e.g., using a root-raised-cosine (RRC) filter to minimize the ICI. See, e.g., US 2012/0301142. However, such filters suppress some data in the transmitted signal, which can lead to the reduction of the transmission rates.
Accordingly, there is a need in the art for a different approach in handling the ICI of the signal transmitted over an optical super-channel formed by a set of sub-channels in the same fiber.